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.. _executor:LocalExecutor:

Local Executor
==============

:class:`~airflow.executors.local_executor.LocalExecutor` runs tasks by spawning processes in a controlled fashion in different modes.

Given that BaseExecutor has the option to receive a ``parallelism`` parameter to limit the number of process spawned,
when this parameter is ``0`` the number of processes that LocalExecutor can spawn is unlimited.

The following strategies are implemented:

- | **Unlimited Parallelism** (``self.parallelism == 0``): In this strategy, LocalExecutor will
  | spawn a process every time ``execute_async`` is called, that is, every task submitted to the
  | :class:`~airflow.executors.local_executor.LocalExecutor` will be executed in its own process. Once the task is executed and the
  | result stored in the ``result_queue``, the process terminates. There is no need for a
  | ``task_queue`` in this approach, since as soon as a task is received a new process will be
  | allocated to the task. Processes used in this strategy are of class :class:`~airflow.executors.local_executor.LocalWorker`.

- | **Limited Parallelism** (``self.parallelism > 0``): In this strategy, the :class:`~airflow.executors.local_executor.LocalExecutor` spawns
  | the number of processes equal to the value of ``self.parallelism`` at ``start`` time,
  | using a ``task_queue`` to coordinate the ingestion of tasks and the work distribution among
  | the workers, which will take a task as soon as they are ready. During the lifecycle of
  | the LocalExecutor, the worker processes are running waiting for tasks, once the
  | LocalExecutor receives the call to shutdown the executor a poison token is sent to the
  | workers to terminate them. Processes used in this strategy are of class :class:`~airflow.executors.local_executor.QueuedLocalWorker`.

Arguably, :class:`~airflow.executors.sequential_executor.SequentialExecutor` could be thought as a ``LocalExecutor`` with limited
parallelism of just 1 worker, i.e. ``self.parallelism = 1``.
This option could lead to the unification of the executor implementations, running
locally, into just one :class:`~airflow.executors.local_executor.LocalExecutor` with multiple modes.